Method for oxalating the galvanized surface of sheet metal

ABSTRACT

A method for forming a zinc oxalate coating on the surface of a strip or sheet of metal covered with a zinc or zinc alloy coating other than zinc/iron coatings, with the aid of an aqueous solution consisting of oxalic acid having a concentration of between 5.10 −3  and 0.1 mole/l, and at least one compound and/or ion of an oxidant zinc metal having a concentration of between 10 −6  and 10 −2  mole/l, and possibly a wetting agent. The inventive method enables sheet metal to be treated at very high speeds without using large amounts of oxidant. It facilitates management of treatment baths. The invention can be used in the lubrication of sheet metal, especially for die stamping.

The invention concerns a method for depositing a zinc oxylate basedlayer onto a zinc based coating, excluding zinc-iron alloys, ofgalvanized sheet metal or metal strips, and the sheet metal or stripsobtained by this method.

Oxalation is a process of surface conversion that has long been appliedto metal surfaces, such as steel, zinc or aluminum, and is intended toform on the surface a oxalate based deposit the pre-lubricationproperties of which facilitate cold forming.

The present invention specifically concerns the treating of galvanizedsurfaces, particularly those of so-called “carbon” steel sheets andstrips. “Carbon steel” is understood as being a steel having aproportion of alloying elements that is distinctly less than what isfound in stainless steels.

Generally, just after the step of oxalating the galvanized surface, thesurface is coated with a thin film of oil (such as QUAKER6130, forexample) in order to provide it with temporary protection againstcorrosion, so that sheet metal treated in this way can be stored forseveral weeks before it is ultimately formed.

The oxalation treatment of galvanized surfaces replaces the usualpre-phosphatizing treatment, and has the advantage of being free of anyharmful consequences on the subsequent operations of assembly andpainting performed at the customers' facilities, because it iscompletely eliminated during the degreasing operation that precedes thephosphatizing.

Thus, patent FR 1 066 186 (SOCIÉTÉ CONTINENTALE PARKER) describes amethod for treating metals such as steel or zinc in a bath of an aqueoussolution composed of:

-   -   1 to 120 g/L oxalic acid, that is, 10⁻² to 0.3 mole/L,    -   0.2 to 50 g/L ferrous chloride FeCl₂ or ferric chloride FeCl₃,        that is, 1.6×10⁻³ to 0.4 mole/L of Fe²⁺ or 1.2×10⁻³ to 0.3        mole/L of Fe³⁺, and    -   5 to 50 g/L of phosphate.

The examples indicate that the treatment times are on the order of oneminute. The application of this solution to a metal surface with thehelp of this oxalate solution containing phosphates makes it possible toobtain coatings that have good adherence to the substrate and thatfacilitate cold forming. However, the presence of phosphates in thesolution is not acceptable from an ecological point of view.

The document U.S. Pat. No. 2808138 (HOECHST) concerns a method oftreating metal surfaces, such as stainless steel or zinc, by an aqueoussolution composed of:

-   -   0.5 to 200 g/L oxalic acid, that is, 5×10⁻³ to 2.2 mole/L,    -   0.5 to 15 g/L ferric ion Fe³⁺, that is, 9×10⁻³ to 0.27 mole/L,        and    -   0.025 to 5 g/L of a soluble compound selected from among        xanthates, dithiophosphoric acid esters, thioglycolic acid and        thioureas. These compounds are no longer acceptable from an        ecological point of view, and also produce a strong,        disagreeable odor. The treatment times are on the order of five        minutes.

Faced with environmental requirements, manufacturers have soughtsolutions for oxalating galvanized surfaces that are moreenvironmentally friendly than those mentioned above. For this reason,phosphate, xanthates, dithiophosphoric acid esters, thioglycolic acidand thioureas contained in oxalation solutions of the prior art make uppart of these compounds, the use of which must be limited bymanufacturers as much as possible, or even eliminated, because of theproblems related to their toxicity and reprocessing.

Only oxalic acid has no toxicity. Manufacturers have therefore developedprocesses implementing only oxalic acid solutions not containing anytoxic compound.

The document U.S. Pat. No. 2,060,365 (CURTIN HOWE CORP.) concerns thetreatment of galvanized surfaces by means of an aqueous solutioncomposed of ferric oxalate Fe₂(C2O₄)₃ (1 to 10%, or 0.05 to 0.5 mole/Lof Fe³⁺) and free oxalic acid in sufficient quantity to inhibit thehydrolysis of the ferric salt. On page 1, column 2, lines 37 to 42, itis indicated that the solution is preferably 4 to 5% ferric oxalate (or0.2 to 0.26 mole/L) and 0.5 to 1% oxalic acid (or 5×10⁻² to 10⁻¹) mole/Loxalic acid. This forms a mixed layer of 66% ferric oxalate and 33% zincoxalate on the galvanized surface, which is not suitable for forming thetreated product. Moreover, in the presence of a corrosion agent such asa base, there is a decomplexation of the ferric oxalate and the ferrichydroxide is obtained according to the following reaction:Fe₂(C₂O₄)₃+6OH⁻→2Fe(OH)₃+3C₂O₄ ²⁻However, the ferric hydroxide has a rusty red appearance that will beunacceptable to customers.

On the other hand, the product resulting from a basic attack of asubstrate coated with a layer of zinc oxalate, that is, zinc hydroxide,has a gray appearance that is not unfavorable.

Because the nature of the elements of a galvanized surface is quitedifferent from those of a carbon or stainless steel surface, thereaction model of oxalation is different. On steel, a layer is obtainedof ferrous oxalate FeC₂O₄ the behavior of which is similar to that ofthe zinc oxalate ZnC₂O₄, that is, it improves the drawability of thesubstrate treated in this way. However, because the oxalation reactionof a ferrous substrate is much slower than the oxalation reaction of agalvanized substrate, this reaction on steel is incompatible withcurrent processing lines. In order to obtain oxalation reaction speedson a ferrous substrate that are compatible with current line speeds, theonly way consists of treating the substrate by anodic polarization. Itis then necessary to work with a treatment facility equipped with anelectrolysis cell and dedicated solely to oxalation, which represents asignificant investment cost. In addition, the range of operation of thistype of process is narrow. The iron must be oxidized to initiate theoxalation reaction, while preventing the simultaneous oxidation of theoxalic acid into CO₂, which considerably limits the accessible range interms of deposit current density and makes the process difficult tocontrol.

The two steps used in the oxalation treatment of galvanized sheet are:

-   1. The dissolving of the zinc: Zn→Zn²⁺+2e⁻; for chemical dissolving    in an acid medium, there would also be: 2H⁺+2e⁻→H₂, or the overall    reaction: Zn+2H⁺→Zn²⁺+H₂.-   2. The complexation of the ions formed and the precipitation of    oxalate complexes of the ZnC₂O₄ type or others.

However, as has already been mentioned, because the formation of an ironoxalate layer is much slower than the formation of a zinc oxalate layer(assuming equal treatment conditions), it is more advantageous for amanufacturer to work with galvanized steel than with bare steel.Moreover, galvanized steel benefits from the corrosion protectionprovided by the zinc layer.

The oxalation of metal surfaces can be implemented by one of thefollowing techniques: immersion, roll-coating or spraying.

The immersion technique consists of moving a strip of galvanized steelat high speed (80 to 100 m/min) through a vat containing a solutioncomposed only of oxalic acid and possibly a wetting agent. When theoxalation treatment is done by immersion, the zinc oxalate deposit onthe galvanized sheet is heterogeneous; in order to obtain a significantpre-lubrication effect on the sheet treated in this way, the thicknessof the zinc oxalate layer must be more than about 0.7 μm, whichcorresponds to a GSM (grams per square meter) on the order of 2 g/m² ofzinc oxalate. Now, with the strip moving at high speed (80 m/min), thetreatment time allowing a layer of zinc oxide to be obtained that wouldimprove the cold formability of the surface treated in this way is veryshort, on the order of 1 to 5 seconds. This is the reason highlyconcentrated solutions of oxalic acid are used, of between 0.3 and 0.8mole/L, so as to obtain zinc oxalate layers on the substrate that aresufficiently thick. However, highly concentrated oxalic acid solutionshave the disadvantage of being aggressive with respect to the treatmentfacility. Indeed, the vats containing the treatment solution aregenerally made of stainless steel. To avoid this problem, much weakersolutions of oxalic acid could be used (concentrations of less than 0.3mole/L). However, the reaction time to obtain a layer of zinc oxalate onthe galvanized surface is much longer, and in this case:

-   -   either the treatment line must be slowed down, to the detriment        of overall production; or    -   much longer treatment vats are used, which represents additional        investment costs, and also is not always possible because of the        amount of space required.

After application of this solution, the sheet can be rinsed and dried inthe standard way. It then receives a fine coating of oil of the typeQUAKER6130 to provide temporary protection against corrosion.

Thus, when working with highly concentrated solutions for a very shorttime, the oxalation reaction of the galvanized surface is very fast andcomplete. The layer of zinc oxalate obtained, whether it is rinsed ornot, has no degradation of properties with respect to temporaryresistance to corrosion. However, there is the problem of theaggressiveness of the acid bath as concerns the facilities.

Furthermore, when working with solutions in weak concentrations for along enough time to have a complete oxalation reaction, there is stillno degradation of properties with respect to temporary resistance tocorrosion (whether the product is rinsed or not). The problem here liesin treatment times that are too long and incompatible with a high speedprocess.

The problem is aggravated even more when working with solutions of weakconcentrations for short time periods. This results in two problems:

-   -   the length of the treatment is not long enough to achieve the        gain expected in drawing, whether the product is rinsed or not;    -   the conversion reaction is not complete; thus, the oxalate        deposit includes oxalic acid that has not reacted with the zinc,        but which is going to react with the coating of oil that the        product is going to receive, if it is not eliminated by rinsing.        In this case, the performance of the oil is seriously        deteriorated. This being the case, whether the product is rinsed        or not, the deposited layer is not thick enough to result in an        improvement in the drawability of the product.

The roll-coating technique consists of moving a strip of galvanizedsteel at high speed (80 to 100 m/min) between two rotating coatingrollers that dip into two vats containing a solution that includes onlyoxalic acid with the possible addition of a wetting agent. In this case,the thickness of the zinc oxalate layer is governed by the quantity ofmaterial deposited by the rollers, and therefore by the roller-sheetdistance, and the time of application of the oxalic acid solution isalso very short, on the order of a second. The application of thetreatment solution by roll-coating without rinsing prior to dryingallows a more homogeneous distribution of the conversion layer thanapplication of the solution by immersion, and GSM's of less than 0.5g/m², and 0.1 g/m² or less, can then be enough to obtain the optimalpre-lubricant properties. In this case, the concentration of the oxalicacid solutions is between 0.3 and 0.8 mole/L, in order to obtain zincoxalate layers on the substrate that are thick enough.

However, the use of highly concentrated oxalic acid solutions hasdisadvantages:

-   -   on the one hand, as has already been seen, concentrated acid        solutions are aggressive with respect to the treatment facility;        the treatment vats are usually stainless steel, and the rollers        for coating the solution are made of rubber or polyurethane;    -   on the other hand, immediately after the roll-coating of the        moving strip, the zinc oxalate coating thus formed is dried by        dryers at 180° C. placed just beneath the treatment vats. The        heat released by the dryers causes an evaporation of the aqueous        solutions of oxalic acid contained in the vats, followed by the        precipitation of the oxalic acid. Thus very quickly solutions        are obtained that have a milky appearance and are unsuitable for        the oxalation reaction desired. The production line must        therefore be shut down, and the vats cleaned and refilled with a        clean oxalic acid solution.

Most industrial lines for coating by roll-coating or immersion do notprovide a rinsing before drying step, because that would considerablyincrease the cost of the oxalation treatment. Indeed, the line wouldhave to be equipped with rinse vats, which is not always possiblebecause of the amount of space required, but especially because therinse effluents would have to be reprocessed. Therefore, the solution touse aqueous compositions with low concentrations (<0.3 mole/L) of oxalicacid, which would make it possible to avoid the above-mentioneddisadvantages, can not be implemented. Because the oxalation reactionbecomes too slow, the oxalic acid does not react completely with thezinc and a layer is deposited that contains, in addition to the zincoxalate (ZnC₂O₄), oxalic acid that has not reacted, and an intermediatecomplex such as Zn(HC₂O₄)₂. These entities react, through their freecarboxylic acid functions, with the oil with which the treated sheetwill subsequently be coated. This affects the temporary resistance tocorrosion of sheet treated in this way.

Although the solutions mentioned above are environmentally friendly,they are very restrictive and unsatisfactory from an economic point ofview, in that they rapidly damage the facilities and require frequentshut-downs of the treatment line.

With respect to the above-mentioned oxalation reaction scheme, itappears that stage 2 oxalation can only occur if stage 1 dissolving hasalready been started, which is a standard, general model for conversiontreatments. In order to increase the speed of oxalation to a levelcompatible with the speed of movement of the steel sheet in industrialfacilities, it is advisable to increase the speed of dissolving the zinc(stage 1) while maintaining the precipitation conditions of the oxalate(stage 2). Thus, if criteria are established concerning the minimumspeed of oxalation and the minimum GSM of the deposit, the range ofconcentration of oxalic acid can be determined, particularly in anexperimental way, that the treatment solution should have in order tomeet these criteria; this range determines the “range of operation” ofthe treatment, which should be as broad as possible to simplify thecontrol of the industrial conditions of surface treatment by oxalation.

A first solution for increasing the speed of oxalation would consist ofcreating more oxidizing conditions by adding large quantities ofoxygenated water or by electrochemical polarization, which iseconomically disadvantageous. U.S. Pat. No. 5,795,661 (BETHLEHEM STEEL)thus describes the advantage of an oxalation treatment for thepre-lubrication of galvanized sheet, particularly within the scope offorming these sheets, by means of an aqueous solution composed of oxalicacid and oxygenated water.

However, the use of large quantities of oxygenated water in industrialfacilities poses process control problems related to the stability ofthe oxygenated water, which is transformed into water and dilutes thebath, as well as serious corrosion and safety problems.

A second solution would consist of decreasing the pH and increasing theconcentration of oxalic acid. Unfortunately, this solution has thedisadvantages of decreasing the “range of operation” described above,seriously complicating the control of the industrial conditions ofapplication of the treatment.

Moreover, the fact has already been mentioned that, if oxalic acidsolutions are used in low concentrations, the oxalation reaction is notfast enough and the oxalic acid does not have time to react completelywith the galvanized surface of the sheet. Thus a layer is obtained of amixture of zinc oxalate, of a Zn(HC₂O₄)₂ type complex and residualoxalic acid. When this surface is subsequently protected temporarilyagainst corrosion by a layer of oil, the oil reacts with the residualacid functions. This results in poor temporary resistance to corrosionof the surfaces thus treated.

The purpose of the present invention is therefore to make available amethod allowing galvanized steel strips to be treated by means ofecological oxalation solutions, so as to obtain deposits of zincoxalates having good pre-lubrication properties (therefore, sufficientthickness), while significantly increasing the speed of oxalation, andwhile avoiding or limiting the above-mentioned disadvantages.

To that end, a purpose of the invention is to form a zinc oxalate layeron the surface of a metal strip or sheet coated with a layer of zinc orzinc alloy, with the exception of zinc-iron alloys, by means of anaqueous oxalation solution containing oxalic acid, characterized in thatsaid solution is an aqueous solution of oxalic acid in a concentrationof between 5×10⁻³ and 0.1 mole/L incorporating at least one compoundand/or one ion of a zinc oxidizing metal in a concentration of between10⁻⁶ and 10⁻² mole/L, and possibly a wetting agent.

In any case, the concentration in oxidizing ions is less than theconcentration threshold at which precipitations of the respective metalare observed.

The invention can also have one or more of the followingcharacteristics:

-   -   the concentration of oxalic acid is preferably between 5×10⁻³        and 5×10⁻² mole/L.    -   the concentration of compounds and/or oxidizing ions of the zinc        in said solution is preferably between 10⁻⁶ and 10⁻³ mole/L.    -   at least one ion is chosen from the group comprised of Ni²⁺,        Co²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mo³⁺, Sn²⁺, Sn⁴⁺.    -   said solution is applied to said galvanized surface without        electrical polarization of said sheet.    -   the GSM of said zinc oxalate layer is between 0.05 and 3 g/m².

A purpose of the invention is also a method of lubricating andtemporarily protecting a galvanized sheet, characterized in that it hasa step of surface oxalation treatment according to the invention,followed by a step of application of a layer of oil.

Preferably, in the implementation of this lubrication process:

-   -   said oil has at least one fatty ester and/or calcium carbonate        in a proportion of 5% or more.

A purpose of the invention is also a method of drawing a galvanizedsheet, characterized in that it includes, prior to the drawing, a stepof lubrication according to the invention.

Finally, a purpose of the invention is a metal strip or sheet that iscoated with a layer of zinc, then coated with a zinc oxalate based layerobtained by the oxalation method according to the invention,characterized in that said oxalate layer has at least 99% zinc oxalate.

The invention will be better understood from the following description,given by way of non-limiting example.

Quite unexpectedly, the inventors have demonstrated that by adding avery small quantity of a compound and/or an ion of a metal that canoxidize zinc in the oxalation solution according to the invention, alayer of zinc oxalate is obtained on the galvanized surface of the steelsheets or strips treated by said oxalation solution, the thickness ofwhich is sufficient to give the sheet or strip thus treated goodtemporary protection against corrosion and good pre-lubricationproperties.

A galvanized surface of a steel sheet or strip is understood as being asurface coated essentially with zinc, or a zinc-based alloy, with theexception for this invention of zinc-iron alloys.

In the case of the oxalation treatment according to the invention of asheet coated with a layer of zinc, the inventors have demonstrated thatthe conversion layer obtained would include at least 99% zinc oxalate.

The concentration in compounds and/or zinc oxidizing metal ions isbetween 10⁻⁶ and 10⁻² mole/L, preferably between 10⁻⁶ and 10⁻³ mole/L.

For a concentration of metal ions of less than 10⁻⁶ mole/L, the effectof these ions on the speed of oxalation is not significant.

For a concentration of metal ions of 10⁻² mole/L or more, the chemicaldeposit is assisted by the carburizing of the metal elementcorresponding to these ions, at the expense of the oxalation desired.

For oxalation baths with an oxalic acid concentration of more than 0.1mole/L, the use of these metal ions is not always essential toaccelerate the oxalation reaction, except, for example, in the case ofapplication by roll-coating to quickly obtain a complete reaction of thetreatment solution with the surface. In particular for oxalation bathswith an oxalic acid concentration of less than 0.05 mole/L, the additionof these ions in low concentration in the treatment solution is aneffective and economical means of obtaining industrially viableoxalation kinetics by immersion. The invention therefore applies tooxalation baths with an oxalic acid concentration of between 5×10⁻³ and0.1 mole/L, preferably between 5×10⁻³ and 5×10⁻² mole/L.

By means of the zinc oxidizing metal ions, even in small concentrations,a very high speed of oxalation is thus obtained, not only when theoxalic acid concentration is less than 0.05 mole/L, but also even whenthe solution does not contain an oxidizer like oxygenated water insignificant quantities and/or even if the sheet is not polarized; thetreatment facility is therefore more economical and easier to operate.

Table I describes oxalation baths of comparable performance. Compared tothe baths of the prior art, it can be seen that the bath according tothe invention has a lower oxalic acid concentration or does not containoxygenated water.

TABLE I Oxalation baths with comparable drawability performancesOxalation Industrial Solution U.S. Pat. No. 5,795,661 PracticesInvention Oxalic 7 to 14 g/L 27 to 72 g/L 9 g/L ≈ 0.1 mole/L acid:Oxygen-  2 to 4 g/L None None ated water: Zinc oxi- None None 10⁻³mole/L (Ni²⁺) dizing metal ions

Preferably, a metal ion is chosen from the group of ions listed in TableII. This table also indicates the value of the normal potential of theoxidation-reduction couple (ion/corresponding metal element or otherion) in volts (V) compared to the Normal Hydrogen Electrode (NHE).

TABLE II Ions usable in oxalation solutions according to the inventionIons Couple Potential Ions Couple Potential Redox V/NHE Redox V/NHE Ni²⁺Ni²⁺/Ni −0.26 Fe³⁺ Fe³⁺/Fe −0.037 Co²⁺ Co²⁺/Co −0.28 Mo³⁺ Mo³⁺/Mo −0.20Cu²⁺ Cu²⁺/Cu +0.34 Sn²⁺ Sn²⁺/Sn −0.14 Fe²⁺ Fe²⁺/Fe −0.44 Sn⁴⁺ Sn⁴⁺/Sn²⁺−0.151

Finally, the oxalation treatment bath can include wetting agents and theinevitable impurities.

In order to apply the treatment solution containing the metal ions so asto obtain a deposit of zinc oxalate on the galvanized surface of thesheet, the standard procedure is used, for example by immersion,spraying or roll-coating; the application stage is followed by a dryingstage. Between the application stage and the drying stage, the treatedsheet can be rinsed.

The optimal composition of the bath (concentrations of oxalic acid andmetal ions) and the morphology of the oxalate-based deposit obtaineddepend on the conditions of application. These conditions are adapted ina known way to obtain the GSM of oxalate-based deposit needed to obtainthe desired properties, for example pre-lubrication properties.

In order to obtain a significant pre-lubrication effect on a galvanizedsheet, when the oxalation treatment is done by immersion, the minimumnecessary thickness is on the order of about 0.7 μm, which correspondsto a GSM on the order of 2 g/m² of zinc oxalate. The application of thetreatment solution by roll-coating without rinsing before drying makesit possible to achieve a more homogeneous distribution of the conversionlayer and GSM's of less than 0.5 g/m², and even GSM's of 0.1 g/m² orless, can then be sufficient to obtain optimal pre-lubricationproperties.

The oxalate-based deposit obtained on the galvanized surface of thesheet offers properties that are comparable to those of standardoxalate-based deposits of the prior art, at least in the following ways:

-   -   comparable pre-lubrication effects: absence of chattering under        friction, appreciable decrease in the coefficient of friction        (>50%) compared to the same oiled sheet without prior oxalation.    -   easy degreasability in an alkaline medium, oxalate deposit easy        to eliminate, making it possible, for example, to carry out a        phosphatizing treatment under very good conditions.

The method according to the invention allows the “range of operation” ofthe treatment to be extended, that is, the range of oxalic acidconcentrations that allow a sufficiently pre-lubrication deposit to beobtained. For example:

-   -   if the range is between 0.3 and 0.8 mole/L oxalic acid without        adding zinc oxidizing ions,    -   the range obtained by adding zinc oxidizing ions according to        the invention is between 5×10⁻³ and 0.8 mole/L.

This effect facilitates the management of the oxalation baths inindustrial applications.

The invention, therefore, makes it possible to obtain oxalate depositson galvanized sheets:

-   -   at higher speeds, without using large quantities of oxidizer        such as oxygenated water and/or without polarization of the        sheet,    -   and/or by using solutions with lower oxalic acid concentrations        than in the prior art.

As illustrated in the examples given below, by random comparativemeasurements of potential of galvanized sheets immersed in differentoxalation solutions:

-   -   In the presence of oxalic acid alone, a slight slowing is        observed before a fully covering layer of zinc oxalate is        obtained, which reflects an inhibition phenomenon in the        formation of a deposit on the galvanized coating; it is also        observed that this slowing becomes shorter as the oxalic acid        concentration is increased.    -   In the presence of zinc oxidizing metal ions according to the        invention, a very significant decrease, and even disappearance,        of the inhibition phenomenon is observed (see FIG. 3).    -   In the presence of metal ions other than zinc reducing agents,        contrary to the invention, an aggravation of this inhibition        phenomenon is observed.

The important activity of the zinc oxidizing ions in low concentrationindicates a catalyzing effect that impedes the temporary inhibition offormation of the oxalate layer.

The oxalation treatment of galvanized sheets according to the inventioncan be used for all of the usual oxalation applications, such as thosedescribed in the introduction, particularly for the pre-lubrication ofthese sheets.

Observed under a scanning electron microscope, the deposit obtained isin the form of cubic crystals, or in the case of thicknesses of lessthan 0.1 μm, in the form of fine flakes. The average size of thesecrystals can be quite different, in particular depending on theapplication conditions of the treatment solution:

-   -   0.1 to 0.5 μm for a deposit applied by roll-coating,    -   0.5 to 0.8 μm for a deposit applied by immersion.

In an analysis by Glow Discharge Spectroscopy (“GDS”) of an amount ofcarbon in different deposits, which serves as a tracer element for theoxalate, it is found that the deposit, according to the invention, hasabout twice the amount of carbon than a deposit made under the sameconditions but without the addition of a zinc oxidizing metal ion to theoxalation solution (analyses based on deposits made with an oxalationsolution containing 0.1 mole/L of oxalic acid).

An analysis of the ejected ions by sputtering and Secondary Ion MassSpectroscopy (SIMS) reveals the presence of zinc oxidizing ions (Ni²⁺)in the deposit, as illustrated in example 3. These ions are notdetectable by x-ray photoelectron spectroscopy on the outer surface ofthe deposit; these ions are no longer detectable in the thickness bychemical analysis.

Compared to deposits obtained without the addition of zinc oxidizingions in the oxalation solution, a greater proportion of oxidized zinc asZn²⁺ was verified by SIMS throughout the deposit, which would explainthe darker color of the deposit according to the invention, andillustrates the greater thickness of the deposited layer.

Other advantages of the method of the invention will appear from thedescription presented below as non-limiting examples of the presentinvention.

Materials:

1) Galvanized Steel Sheet Used:

-   -   USICAR™ galvanized steel sheet, coated by electroplating with a        zinc layer about 7.5 μm thick, degreased in an alkaline medium.

2) Oxalation Bath:

-   -   Oxalic acid concentration: variable.    -   Nature and concentration of metal ions added: variable.    -   Other components: none.

Methods:

1) Conditions Under Which the Deposit was Made:

-   -   Bath temperature: unless otherwise indicated, ambient        temperature (about 25° C.).    -   Application method: immersion followed by rinse with        decarbonated water, or by roll-coating without rinsing before        drying.    -   Drying method: hot air.    -   Surface density of the dried deposit obtained: unless otherwise        indicated, 2 g/m² by immersion (or about 0.7 μm), 0.1 to 0.3        g/m² by roll-coating.

2) Evaluation of the Pre-lubrication Effect:

-   -   The tribological behavior of surfaces of galvanized steel test        samples treated and untreated by oxalation was evaluated by        measuring the coefficient of friction as follows:    -   before the friction test, the test sample is first oiled in the        usual way, unless otherwise indicated, with QUAKER 6130 oil,    -   the friction test is done with a standard flat-on-plate        tribometer under a clamping pressure increasing from 0 to        800×10+5 Pa; the measurement used generally corresponds to the        average of the friction coefficients measured during the test.

3) Evaluation of the GSM of the Oxalate Base Deposit:

-   -   This is done in two steps:    -   with a galvanized steel test sample treated by oxalation, the        first step is to dissolve the deposit and the underlying zinc        layer,    -   using the solution obtained, the second step is to measure the        quantity of oxalic acid contained in the solution.    -   This quantity is compared to the treated surface area to obtain        the GSM.    -   1st Step: Using a set-up with three electrodes (the treated        galvanized steel test sample, a stainless steel secondary        electrode, and a saturated calomel electrode “SCE” for        reference), electro-dissolution of the deposit and of the zinc        coating is carried out by applying a potential of −800 mV/SCE to        the treated galvanized steel test sample; when the current        ceases, all of the zinc is considered to have gone into solution        in the electrolyte; in the solution thus obtained, which has a        very acid pH, the zinc oxalate is considered to be completely        decomplexed according to the reaction:        2H⁺+ZnC₂O₄→Zn²⁺+H₂C₂O₄    -   2nd Step: In the solution obtained, a few drops of manganese        sulfate are added to catalyze the oxidation-reduction reaction.        The amount of H₂C₂O₄ oxalic acid is then measured by a solution        of potassium permanganate of known normality, according to the        reactions:        oxidation of the oxalic acid: H₂C₂O₄→2CO₂+2H⁺+2e″        reduction of the permanganate: MnO_(4″)+8H⁺+5e″→Mn²⁺+4H₂O        -   While the permanganate is being added, the change in the            solution's potential is measured between a platinum            electrode and a saturated calomel electrode. The jump in            potential corresponds to the equivalent amount according to            the formula N_(o)×V_(o)=N×V_(eq)., where No is the normality            of the oxalic acid solution to be titrated, V_(o) is the            volume of this solution to be titrated, N is the normality            of the permanganate titration solution, and V_(eq). is the            volume of this titration solution added to bring about the            jump in potential.        -   Starting with N_(o), from the volume of the            electro-dissolution solution, from the treated surface area            of the test sample, the surface density of the oxalate-based            deposit on the initial test sample and/or the average            thickness of this deposit is calculated in a known way.

4) Characterization of Corrosion from Moisture/Heat (DIN Standard 51160)

-   -   The samples to be tested are placed in a climatic condition        reproducer corresponding to DIN 51160, which simulates the        corrosion conditions of an outer turn of a spool of sheet metal        or of a sheet cut to length during storage.    -   The detail of the cycle (one cycle =24 hours) under controlled        moisture/heat conditions is described below:        -   8 hours at 40° C. and 100% relative humidity        -   16 hours at ambient temperature and humidity    -   The test samples are individually suspended vertically.    -   Visual observation of the test samples makes it possible to        quantify the degradation of the coating by the appearance of        white rust, based on the number of successive cycles of        exposure. The ratings are stopped when at least 10% of the total        surface of the test sample show signs of white rust.

COMPARATIVE EXAMPLE 1

The purpose of this example is to illustrate, with reference to FIG. 1,the change in immersion oxalation speed of a galvanized sheet based onthe oxalic acid concentration of the treatment bath and/or based on thetemperature of the bath.

FIG. 1 represents the variation in thickness of an immersion deposit ofzinc oxalate (μm) as a function of the duration of the oxalationtreatment, i.e., the duration of immersion(s), for different oxalic acidconcentrations of 0.1, 0.3, 0.5 and 0.8 mole/L, and two temperatures 25°C. and 50° C., that is, a total of eight curves (the curves for 0.1mole/L at 25° C. and 50° C. are merged).

The results obtained are shown in FIG. 1 for the following treatmentsolutions and temperatures:

A: [H₂C₂O₄]=0.1 mole/L at 25° C. or 50° C.

B: [H₂C₂O₄]=0.3 mole/L at 25° C.

C: [H₂C₂O₄]=0.5 mole/L at 25° C.

D: [H₂C₂O₄]=0.8 mole/L at 25° C.

E: [H₂C₂O₄]=0.3 mole/L at 50° C.

F: [H₂C₂O₄]=0.5 mole/L at 50° C.

G: [H₂C₂O₄]=0.8 mole/L at 50° C.

In order to obtain a significant pre-lubrication effect in the case ofthe immersion application, routine tests have shown that the thicknessof the zinc oxalate deposit should be about 0.7 μm or more.

According to the curves in FIG. 1, for a duration of treatment of 0.5sec., it is observed that this thickness of 0.7 μm is reached:

-   -   at 25° C., as soon as [C₂O₄ ²⁻]≧0.8 mole/L,    -   at 50° C., as soon as [C₂O₄ ²⁻]≧0.3 mole/L.

It is evident, therefore, that to obtain a high oxalation speed withoutelectrical polarization of the sheet to be treated and without oxidizingthe zinc at a high concentration, solutions should be used with anoxalic acid concentration that is considerably greater than 0.1 mole/L,at least: 0.3 mole/L at 50° C., 0.8 mole/L at 25° C.

FIGS. 2A, 2B and 3 illustrate examples 1 and 2. The random potential(y-axis) (measured in mV with respect to a Saturated Calomel Electrode:“SCE”) of a galvanized steel sheet as a function of time (s) (x-axis),measured from the moment of immersion of the sheet (zero time) bychronopotentiometry at nearly-nul current.

EXAMPLE 1

The purpose of this example is to illustrate, according to theinvention, the effect of adding, in very weak concentration, Ni²⁺ ionsto the treatment solution on the oxalation speed of the galvanizedsheet, using here—again by immersion—different treatment solutions at25° C. containing the same proportion of 0.5 mole/L oxalic acid. TheNi²⁺ ions are zinc oxidizing.

In order to continuously evaluate the oxalation speed of an oxalationsolution, the potential of the galvanized steel sheet is randomlymeasured starting at the moment (zero time) immersion of the sheet insaid solution begins. The steel sheet electrode is in the form of acircular disk with surface area of 0.2 cm². During the measurement, theelectrode is driven in rotation at 1250 revolutions per minute.

The results obtained are shown in FIG. 2A for the following treatmentsolutions:

-   -   C=comparative: [H₂C2O₄]=0.5 mole/L with no addition of ions,    -   A: [H₂C2O₄]=0.5 mole/L and [NiCl₂]=10⁻³ mole/L    -   B: [H₂C2O_(4]=0.5) mole/L and [NiCl₂]=10⁻⁴ mole/L

The curve C (comparative) is for a solution of the prior art, withoutthe addition of zinc oxidizing ions. It shows a first phase of a steadyincrease of the potential for up to about 100 seconds, followed by asecond phase of slow, steady and slight decrease. In the first phase, itcan be seen that the oxalation speed is very weak in the first moments,then steadily increases (increase of the slope of the curve). This veryweak oxalation reveals a temporary inhibition phenomenon of thegalvanized surface that the invention specifically makes it possible tolimit.

Curves A and B are for solutions according to the invention, containingzinc oxidizing ions. They show that the oxalation is nearlyinstantaneous, which indicates that very small quantities of Ni²⁺ ionsadded to the solution make it possible for this inhibition phenomenon tobe limited or even eliminated, for the reactivity of the galvanizedsurface to be to considerably increased, and for the oxalation speed tobe very strongly increased.

FIG. 2B shows that this effect results from a synergy between the C₂O₄²⁻ ions and the Ni²⁺ ions; the results listed are for the followingtreatment solutions:

-   -   A: [H₂C2O_(4]=0.5) mole/L and [NiCl₂]=0.001 mole/L    -   I: a solution that contains only a weak concentration of zinc        oxidizing ions, with no oxalic acid: [NiCL₂]=0.001 mole/L.

From the results listed for this figure, it can be clearly seen that theNi²+ions alone do not have an effect that is comparable to that of theC₂O₄ ²⁻+Ni²⁺ ions.

EXAMPLE 2

The purpose of this example is to illustrate that only ions that arezinc oxidizing, even in small concentrations, produce this synergisticeffect and make it possible to increase the oxalation speed.

As in example 1, random measurement of the potential of the samegalvanized steel sheet immersed in the treatment solution to beevaluated is used.

In order to better differentiate the effect of the metal ions added tothe solution on the speed of oxalation, the solutions used here containonly 0.05 mole/L of oxalic acid, again at 25° C.; For all of thesolutions (except for the reference B), the concentration of added ionsis 10⁻³ mole/L.

FIG. 3 shows the curves of change in random potential pertaining to thefollowing treatment solutions:

A: [H₂C₂O₄]=5×10⁻² mole/L and [MnCl₂]=10⁻³ mole/L

B =reference: [H₂C₂O₄]=5×10⁻² mole/L

C: [H₂C₂O₄]=5×10⁻² mole/L and [NiCl₂]=10⁻³ mole/L

D: [H₂C₂O₄]=5×10⁻² mole/L and [CoCl₂]=10⁻³ mole/L

E: [H₂C₂O₄]=5×10⁻² mole/L and [CuCl₂]=10⁻³ mole/L

The Cu²⁺, Co²⁺ and Ni²⁺ ions are zinc oxidizing and are therefore usablefor the invention, while the Mn²⁺ ions are not zinc oxidizing and arenot usable for the invention.

The normal oxidation-reduction potentials of the couples (metal ions/-corresponding metal with respect to the normal hydrogen electrode are:

-   -   E₀ (Cu²⁺/Cu)=+0.34 V    -   E₀ (Ni²⁺/Ni)=−0.26 V    -   E₀ (Co²⁺/Co)=−0.28 V    -   For reference: E₀ (Zn²⁺/Zn)=−0.76 V    -   E₀ (Mn²⁺/Mn)=−1.18 V

By comparing the curves of FIG. 3 and these oxidation-reduction values,it clearly can be seen that the acceleration action of the metal ion onthe oxalation speed is particularly pronounced when this ion is zincoxidizing. The reverse is true for the Mn²⁺ ion, which is a reducingagent and outside the field of the invention. It has a slowing effect onthe oxalation.

EXAMPLE 3

The purpose of this example is to find in which concentration domain thezinc oxidizing ion added to the treatment solution is effective incatalyzing and accelerating the oxalation of the galvanized surface.

As in example 2, the curves show the random potential of a galvanizedsteel electrode in solutions having 0.05 mole/L of oxalic acid anddifferent concentrations of NiCl₂ spaced out between 10⁻⁷ and 10⁻¹mole/L. It can be seen that the catalytic effect of the Ni²⁺ ions isproduced as soon as the NiCl₂ concentration reaches 10⁻⁶ mole/L. Thiseffect is always observed for greater concentrations, up to 10⁻² mole/L.Beyond that concentration, a chemical nickel deposit can be observedwith the naked eye.

EXAMPLE 4

The purpose of this example is to illustrate the physical-chemicalcharacteristics of the deposit according to the invention thatdifferentiate it from an oxalation deposit done according to the priorart (reference).

The analytical method used to determine these differences is SecondaryIon Mass Spectroscopy (“SIMS”) of the ions ejected from the oxalatedeposit by ionic bombardment.

FIG. 4 illustrates, from top to bottom, the profiles of Ni⁺ ₅₈, O⁻ ₁₆and ZnO⁺ ₈₀ obtained by Secondary Ion Mass Spectroscopy (“SIMS”) on anoxalate based deposit produced according to the invention (A curves) andon a deposit produced under the same conditions but without the additionof oxidizing metal ions (B curves); the curves indicate the intensity ofthe signal as a function of the sputter time (0 to 25 minutes), that is,as a function of the depth from the outermost surface.

FIG. 4, divided into three parts that are referenced, from top tobottom, “Ni”, “O” and “ZnO” show the results obtained respectively forthree types of ions: Ni⁺ ₅₈, O⁻ ₁₆ and ZnO⁺ ₈₀; on each part two curvesor profiles are indicated: curves A for a deposit produced according tothe invention in the presence of nickel ions, curves B for a referencedeposit produced under the same conditions but without the addition ofnickel ions.

The sputter time is extended to 25 minutes and corresponds to a depth onthe order of about 1 to 1.5 μm.

It is determined from these results that:

-   -   the nickel added to the oxalation bath is present in the        thickness of the deposit made in the presence of Ni²⁺ at a        concentration at least 5 times greater than in the thickness of        the reference deposit; the nickel detected in the reference        deposit corresponds to the nickel in the inevitable impurities        present in the bath.    -   the addition of Ni²⁺ to the oxalation bath increases the        proportion of zinc in Zn²⁺ oxidized state in the deposit, which        confirms that this addition promotes the dissolving and        oxidation of the zinc (as Zn²⁺) of the surface to be treated and        makes it possible to increase the thickness of the deposited        layer.

EXAMPLE 5

The purpose of this example is to illustrate the possible synergiesbetween the oxalate base deposit and a lubrication oil, particularly inthe case where this oil contains fatty esters and/or calcium carbonate.

Fatty esters are standard components of lubricating oils. Calciumcarbonates are standard additives for these oils, dispersed andemulsified in the oil phase, generally with the aid of alkyl sulfonatesor alkyl-aryl sulfonates. The usual term for this mixture is “overbasedcalcium sulfonate.”

The QUAKER 6130 oil used in the procedure to evaluate thepre-lubrication effect (point 2, METHODS paragraph above) contains, inaddition to olefinic or paraffinic mineral oil, both of the followingcomponents: about 16% fatty esters and about 5% calcium carbonate.

Friction tests are carried out (point 2, METHODS paragraph above, inthis instance with a constant clamping pressure of 400×10⁺⁵ Pa) ongalvanized test samples that have not been treated by oxalation, and ontest samples treated by roll-coating according to the invention so as toobtain an oxalate base with a GSM on the order of 0.3 g/m².

Prior to the friction test, the samples are coated:

-   -   with pure mineral oil not containing any fatty esters or calcium        carbonate (SHELL 2881 oil);    -   or QUAKER 6130 oil,    -   or a layer of calcium carbonate, applied by roll-coating a        solution of overbased calcium sulfonates diluted in hexadecane;    -   or a layer of fatty ester, also applied by roll-coating a        solution of methyl oleate (fatty ester) diluted in hexadecane;

For the friction tests, the minimum friction coefficient (μ_(min)) atthe end of the test is measured; the results obtained are shown in TableIII.

TABLE III Friction Results Oil Used μ_(min) Untreated SHELL 2881 oil0.19 surface Calcium carbonate 0.25 Fatty ester 0.25 QUAKER 6130 oil0.16 Treated SHELL 2881 oil 0.14 surface Calcium carbonate 0.1(invention) Fatty ester 0.1 QUAKER 6130 oil 0.09

It is obvious, therefore, that an oxalate based deposit offers a muchgreater pre-lubrication effect with an oil having at least one fattyester and/or calcium carbonate in a proportion of 5% or more, than withan oil that does not contain these components. The results clearly showthe synergy of the oxalate based deposit with each of these components.

EXAMPLE 6

The purpose of this example is to illustrate that galvanized sheetstreated according to the invention (application of the solutionaccording to the invention by the roll-coating technique) then coatedwith a thin film of QUAKER 6130 oil have good properties for drawing aswell as temporary corrosion [protection].

TABLE IV Results of Behavior under Controlled Moisture/Heat Conditionsand to Drawing Behavior under controlled moisture/heat conditions;number of GSM cycles before (g/m² ± appearance of METHODS 0.02) 10%white rust Drawing USICAR ™ reference — 20 cycles Poor USICAR ™ treatedwith H₂C₂O₄ 0.2 Poor: 2 cycles Excellent at 0.1 M USICAR ™ treated withH₂C₂O₄ 0.2 Poor: 2 cycles Excellent at 0.05 M USICAR ™ treated with 0.05M 0.23 Good: Excellent H₂C₂O₄ + 10^(−[illegible]) M CuCl₂ 20 cycles Notrinsed USICAR ™ treated with 0.05 M 0.21 Good: Excellent H₂C₂O₄ +10^(−[illegible]) M CuCl₂ 18 cycles Rinsed USICAR ™ treated with 0.05 M0.16 Good: Excellent H₂C₂O₄ + 10^(−[illegible]) M CuCl₂ 24 cycles Notrinsed USICAR ™ treated with 0.05 M 0.18 Good: Excellent H₂C₂O₄ +10^(−[illegible]) M CuCl₂ 17 cycles Rinsed USICAR ™ treated with 0.1 M0.21 Good: Excellent H₂C₂O₄ + 10^(−[illegible]) M CuCl₂ 20 cycles Notrinsed USICAR ™ treated with 0.1 M 0.19 Good: Excellent H₂C₂O₄ +10^(−[illegible]) M CuCl₂ 18 cycles Rinsed USICAR ™ treated with 0.1 M0.21 Good: Excellent H₂C₂O₄ + 10^(−[illegible]) M CuCl₂ 20 cycles Notrinsed USICAR ™ treated with 0.1 M 0.20 Good: Excellent H₂C₂O₄ +10^(−[illegible]) M CuCl₂ 16 cycles Rinsed (Note: In this table, theunit mole/L is indicated by M.)

These results show that galvanized sheets (USICAR™) treated with oxalicacid alone and in weak concentrations (0.1 mole/L and 0.05 mole/L), andwith a GSM of 0.2 g/m², have good drawing properties, but poorproperties with respect to moisture and heat. The poor properties withrespect to moisture and heat are probably related to the fact that theoxalation reaction used does not lead just to the formation of a ZnC₂O₄type complex, but to the deposit of a mixed layer composed, in additionto said complex, of oxalic acid that has not reacted and/or of anothercomplex of the Zn(HC₂O₄)₂ type, which also has acid functions. In thepresence of oil, the free acid functions of the layer would react withthe sulfonate functions of the oil (corrosion inhibitor compounds) by anacidobasic reaction. Because of this, the corrosion inhibitingproperties of the oil would be depleted and it would no longer be ableto provide its corrosion protection function.

Moreover, the addition of very small quantities of an activating agentlike Cu²⁺ to an oxalation bath in weak concentration (10⁻³ or 10⁻⁴mole/L) makes it possible to produce deposits on the treated galvanizedsurface that are nearly free from a soluble phase. Indeed, the resultsclearly show that there is no significant difference in GSM betweenrinsed and unrinsed samples.

In addition, from solutions according to the invention, that is, thosefor which the oxalic acid concentration varies from 0.05 mole/L to 0.1mole/L and the CuCl₂ concentration varies from 10⁻³ to 10⁻⁴ mole/L, theGSM's of the zinc oxalate layers deposited on the treated galvanizedsurface are close to the target GSM (0.2 g/m²), and lead to goodproperties with respect to heat and moisture, as well as excellentdrawing properties.

1. A method for forming a zinc oxalate layer on the surface of a metalstrip or sheet coated with a layer of zinc or zinc alloy, with theexception of zinc-iron alloys, comprising applying an aqueous oxalationsolution to the surface of said metal strip or sheet, wherein saidaqueous oxalation solution consists of oxalic acid in a concentrationbetween 5×10⁻³ and 0.1 mole/L, at least one compound and/or one ion of azinc oxidizing metal in a concentration of between 10⁻⁶ and 10⁻² mole/L,water, impurities and optionally a wetting agent.
 2. The methodaccording to claim 1, wherein said concentration of oxalic acid isbetween 5×10⁻³ and 5×10⁻² mole/L.
 3. The method according to claim 1,wherein the concentration of compounds and/or ion of a zinc oxidizingmetal in said solution is between 10⁻⁶ and 10³¹ ³ mole/L.
 4. The methodaccording to claim 1, wherein said at least one ion is chosen from thegroup consisting of Ni²⁺, Co²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mo³⁺, Sn²⁺, and Sn⁴⁺.5. The method according to claim 1, wherein said aqueous oxalationsolution is applied to said surface of a metal strip or sheet coatedwith a layer of zinc or zinc alloy without electrical polarization ofsaid metal strip or sheet.
 6. The method according to claim 1, whereinthe GSM of said zinc oxalate layer is between 0.05 and 3 g/m².
 7. Amethod of lubricating and temporarily protecting a metal strip or sheetcoated with a layer of zinc or zinc alloy, with the exception ofzinc-iron alloys, comprising forming a zinc oxalate layer on the surfaceof said metal strip or sheet coated with a layer of zinc or zinc alloyaccording to claim 1, followed by applying a layer of oil.
 8. The methodaccording to claim 7, wherein said oil has at least one fatty esterand/or calcium carbonate in a proportion of 5% or more.
 9. A method ofdrawing a metal strip or sheet coated with a layer of zinc or zincalloy, with the exception of zinc-iron alloys, comprising, prior todrawing, lubricating and temporarily protecting said metal strip orsheet coated with a layer of zinc or zinc alloy, with the exception ofzinc-iron alloys, according to claim
 7. 10. The method according toclaim 1, wherein said aqueous oxalation solution is applied to thesurface of said metal strip or sheet by immersion, spraying or rollcoating followed by drying.
 11. The method according to claim 10,further comprising rinsing between the application of said aqueousoxalation solution and the drying.
 12. The method according to claim 1,wherein said aqueous oxalation solution is applied to the surface ofsaid metal strip or sheet by roll coating followed by drying, withoutrinsing between the application of said aqueous oxalation solution andthe drying.
 13. The method according to claim 1, wherein said oxalatelayer has at least 99% zinc oxalate.
 14. A method for forming a zincoxalate layer on the surface of a metal strip or sheet coated with alayer of zinc or zinc alloy, with the exception of zinc-iron alloys,comprising applying an aqueous oxalation solution to the surface of saidmetal strip or sheet, wherein said aqueous oxalation solution consistsessentially of oxalic acid in a concentration between 5×10⁻³ and 0.1mole/L, at least one compound and/or one ion of a zinc oxidizing metalin a concentration of between 10⁻⁶ and 10⁻² mole/L, water, andoptionally a wetting agent.